Image reproduction device



Jan. 19, 1960 F. K. COLLINS ETAL 2,922,072

7 IMAGE REPRODUCTION DEVICE Filed Dec. 5, 1957 INVENTORS FLOYD KlCOLL/IVS 0554/? A. DRAKE zmw ATTOR N EY IMAGE REPRODUCTION DEVICE FloydK. Collins and Oscar A. Drake, Seneca Falls, N.Y., assignors, by mesneassignments, to Sylvania Electric Products Inc., Wilmington, DeL, acorporation of Delaware Application December 5, 1957, Serial No. 700,848

1 Claim. 01. 315-16) tial applied to these electrodes is of a constantvalue since dynamic focusing using a varying potential is complicatedand expensive to achieve. needed to focus the electron beam or beams insuch tubes has heretofore been dependent upon the beam current. Sincethis current is being continuously modulated or varied during tubeoperation, the beam is not correctly focused over the major portions ofthe reproduced picture or display regardless of the arbitrary focusingvoltage selected.

Other aspects of the beam forming problems which are interrelated withfocusing concerns the actual shape of the beam spots and the haze whichsurrounds it when viewed at the impinging position on the screen. Veryoften, electrode voltages capable of producing the best focusingcharacteristic with a given tube structure also produces an undesirablydistorted and hazy beam spot.

Accordingly, an object of the invention is the provision ofsubstantially constant focusing for cathode ray tubes.

A further object is the provision of substantially constant focusingwith the maintenance of a substantially undistorted electron beam spothaving a minimum amount of haze.

A further object is to improve the fabrication of cathode ray tubes andthe electrode gun mounts employed therein.

The foregoing objects are achieved in one aspect of the invention by theprovision of an electrostatic focus type cathode ray tube employing ascreen grid spaced from an anode having an aperture formed therein.Substantially constant beam focus with optimum beam spot quality isachieved by providing a specific ratio of the anode aperture to thespacing between the grid and anode when these electrodes have givenprescribed voltages imposed thereon relative to one another.

For a better understanding of the invention, reference is made to thefollowing description taken in conjunction with the accompanyingdrawings in which:

Fig. 1 is a partially sectioned plan view of a cathode ray tube;

Fig. 2 diagrammatically illustrates the electrode mount configurationsand the beam shape for an electrode mount of the type adapted to beemployed with the cathode ray tube shown in Fig. 1; and 1 Fig. 3 is agraphical representation of the manner in which the focusing voltagebehaves as the cathode or beam current is varied.

Referring to the drawings, a cathode ray tube 11 is shown comprising anenvelope 13 having an electron gun 15 disposedin the neck portion 17thereof. The electron gun provides the source, control, acceleration andThe voltage 2 focusingof the electron beam 19, which scans screen 21under the influence of the magnetic fields provided by deflection coils23.

The electron gun 15 comprises a cathode 25, control grid-27, screen grid29 and a first anode 31 arranged substantially along the axis of thetube to provide the source, control, pre-focusing and acceleration ofbeam 19. An electrostatic focusing assembly comprising a first lens cup33, lens ring 35 and second lens cup 37 is shown mounted upon commoninsulating support rods 39 with the other electrodes. If desired, thefocusing electrodes may be mounted separately, and the'first anode 31and first lens cup 33 may be fabricatedas one part, as shown, or asseparate parts. During operation of tube '11,'the video signal isgenerally coupled to cathode 25 to provide a bias relative to grid 27. Avoltage V of approximately 300 volts may: be imposed upon screen grid 29while first anode 31, first lens cup 33 and second lens cup 37 may becoupled to voltage V of 16 kv. or over. Generally, lens ring 35 isoperated at a voltage V, up to 300 volts. The lower voltages may beapplied to their respective electrodes through lead-in base connections(not shown) to the tube while the high potentials are applied throughinternal connections (not shown) with the second anode conductivecoating 41.

Electron beam 19 is focused in the manner shown in Fig. 2. The electronsemitted from cathode 25 proceed through the apertures in grids 27 and 29to a crossover point 30. The electrostatic field between anode 31 andgrid 29 forms the electrons into a beam which extends through theapertures in lens cups 33 and 37. The pri mary focusing is achieved bythe electrostatic fields or lens existing in the vicinity of lens ring35. Beam 19 is thereby focused to provide a small beam spot on screen21.

It has been found that spacing S between the internal surface of thebase 26 of cup-shaped grid 29 and the edge of tubular anode 31 disposedadjacent the open mouth portion 28 of grid 29 along with the diameter Dof the aperture in anode 31 are instrumental in achieving constant focusof beam 19 for a wide range of beam current values with minimumdistortion and haze. A ratio D/S greater than 1.3 has been found toproduce the desired results. Referring to Fig. 3, it can be seen that astructure of the type described herein provides constant focus voltageE, for a wide range of beam currents I It has been found that as thespacing S becomes too small, e.g., below .050 inch, the curve loses itslinearity,

at all values of beam currents.

The ratio of D/S which has been found satisfactory is between 1.3 and6.0 when the voltage V on anode 31 is at least 10 times greater than thevoltage V imposed on grid 29. Results have indicated that for anodevoltages even below 6 k.v., the ratio still provides good focusingcharacteristics. I

Excellent results have been attained by using a spacing S of .145 inchwith an anode aperture diameter D of .235 inch when V and V are 300volts and 16 k.v. respectively. Varying within reasonable limits thelength and width of anode 31 and the length and width of grid 29 hasbeen found not to materially affect the'focusing characteristics of thetube. 1

Spot hazing may be improved somewhat by varying the aperture diameter Win lens cup 33 in accordance with the length L of the integral structurecomprising anode 31 and lens cup 33.- Generally, it has been found thatthe aperture diameter should vary with the length L of the anode cupcylinder to provide minimum spot hazing. For instance, with a length Lof .650 inch, aperture size W is found to be approximately .060 inch,i.e. L/ W=l0.8, whereas fora value L of .775 inch, a satisfactoryaperture diameter W is .090 inch, i.e. L/W=8.6.

Although the electrodes described herein generally have cylindricalforms, it is to be understood that the invention is not restricted tosuch an electrode. For instance, grid 29 may be formed as an apertureddisc, if desired. Also, anode 31 may "be formed as a straight cylinderand may be fabricated separately from the first lens cup.

Although several embodiments of the invention have been shown anddescribed, it will be apparent to those skilled in the art that variouschanges and modifications may be made therein without departing from thescope of the invention as defined by the appended claim:

What is claimed is:

An image reproduction device electrode structure employing a pluralityof axially aligned electrodes comprising, in order, a cathode providingan electron beam, a control grid, a'screen grid havingsa base portionand an open mouth portion, a tubular anode spaced from said base portiona distance S ranging from .050 to .200 inch formed with an aperture ofdiameter D which is smallerthan said open mouth portion, and anelectrostatic beam focusing lens comprising a first lens cup integralwith said anode having a re-entrant wall portionformed to provide anaperture of diameter W, the integral anode and first lens cup having anoverall length L, a lens ring, and a second lens cup, the electrodestructure providing substantially constant focusing of said beam whenthe ratio D/S ranges from'1.3 to 6.0, the ratio of L/ W rangesfrom8.6.to- 10.8, and the voltage imposed upon said anode is greater thanten times the voltage imposed upon said screen grid.

References Cited in the file of this patent UNITED STATES PATENTSKnochel et al. Sept. 23, 1958

